In the 14th division of the Drosophila embryo, the rapid maternally driven cycles end and zygotic regulation begins. This transition, referred to as the mid-blastula transition (MBT), coordinates a multitude of cellular events to ensure developmental fidelity. This proposal seeks to characterize how titration of cyclin:Cdk activity and changes in chromatin modifications influence the MBT. By genetically altering cyclin:cdk activity, we will monitor: cell cycle timing by real time flourescence microscopy, replication dynamics by injecting labeled nucleotides, and the transcription of key genes by in situ hybridization. Continued characterization of the "histone code" will define changes in chromatin modifications at the MBT and injecting deacetlyase inhibitors into staged embryos will directly test how chromatin state influences the MBT and replication. Finally, the relationship between cyclin:Cdk activity and histone modifications will be examined using mutants of cyclin:Cdk regulation, non-degradable cyclins, and Cdk inhibitors. Understanding the MBT and cell cycle transitions is important not only in discerning basic development, but in elucidating how defects in this process lead to abnormalities, including cancer and birth defects.